In a UMTS (Universal Mobile Telecommunications System) network, for the purposes of improving spectral efficiency and improving the data rates, system features based on W-CDMA (Wideband Code Division Multiple Access) are maximized by adopting HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access). For this UMTS network, for the purposes of further increasing high-speed data rates, providing low delay and so on, long-term evolution (LTE) has been under study (see, for example, Non-Patent Literature 1).
In a third-generation system, it is possible to achieve a transmission rate of maximum approximately 2 Mbps on the downlink by using a fixed band of approximately 5 MHz. Meanwhile, in a system of the LTE scheme, it is possible to achieve a transmission rate of about maximum 300 Mbps on the downlink and about 75 Mbps on the uplink by using a variable band which ranges from 1.4 MHz to 20 MHz. Furthermore, in the UMTS network, for the purpose of achieving further broadbandization and higher speed, successor systems of LTE have been under study (for example, LTE-Advanced (LTE-A)). For example, regarding LTE-A, there is a plan to expand the 20-MHz maximum system band of LTE specification to approximately 100 MHz.
In a system of the LTE scheme (LTE system), downlink control information is transmitted from a base station apparatus to mobile station apparatuses on PDCCHs (Physical Downlink Control Channels). PDCCHs include a PDCCH (UE-common PDCCH) which all mobile station apparatuses need to receive at the same time, and PDCCHs (UE-specific PDCCHs) which mobile station apparatuses have to receive individually. In order to detect these PDCCHs while reducing load in mobile station apparatuses, the LTE system defines a UE-common search space, which is placed in a common position between all mobile station apparatuses and which includes the above UE-common PDCCH, and a UE-specific search space, which is placed in a separate position for each mobile station apparatus and which includes the above UE-specific PDCCHs. Amongst the CCEs (Control Channel Elements), which are the fundamental units to which PDCCHs are assigned, the UE-common search space is placed in CCEs placed in the top part. On the other hand, the UE-specific PDCCH is placed randomly based on mobile station apparatus identification information (UE-ID) and subframe numbers.